Operation of lifting cranes requires careful attention and control in order to avoid various undesirable developments and conditions. One condition generally to be avoided during operation of a lifting crane is what is known as two blocking. This occurs when the lifting hook or the lifting hook block of a crane is raised to the point that it comes into contact with the boom nose sheave or some other portion of the boom nose. Such contact can result in stresses which are too great for various structural elements of the crane, or may result in unstable operation.
It is known to provide cranes with anti-two blocking devices which guard against such a condition and/or warn a crane operator of actual or impending two blocking. A typical such device may comprise a mechanical switch which remains in a first position during general operation of the crane, but is moved into a second position by contact with the hook block if the hook block approaches the boom nose too closely. A signal resulting from the change in state of the switch can be used to warn an operator of possible two blocking, or the signal may be provided to a control system for managing operation of the crane automatically. Non-contact type anti-two block sensors are also known. Such sensors detect the approach of the lifting hook block to the boom nose by non-contact means, such as use of infrared technology.
As noted, anti-two blocking devices and switches are normally located proximate the boom nose of a lifting crane. As a result, power for the device typically must be run, via a wire and/or conduit, from a power source at the base of the crane, along the crane boom, to the region of the boom nose in order to provide operating power. In cranes comprising extensible booms, this requires complex linkages, conduits, or other structures to accommodate the fact that the power must run along a path that is variable in length, and between elements which may be articulated with respect to each other in various ways. Such arrangements can be somewhat costly and difficult to construct and maintain.
It is known, in connection with anti-two block devices, to use a storage battery located proximate to the boom nose for providing operating current to the device. Batteries have an inherent shortcoming in that they have a finite service life. Accordingly, reliance upon a battery to power, an anti-two block device presents a risk that a sensor of the anti-two block device will simply not have operating power when its operation is needed. Additionally, upon startup of a crane, it may take some amount of time before it may be recognized if the anti-two block sensor is functioning properly or if it is not functioning due to lack of operating current, or for any other reason. Should this occur, and if the crane is operated during any period of time when the anti-two block sensor is not functioning properly, it is possible for the crane to two-block without warning or control.